There are a number of different cellular telephone standards, such as GSM, UMTS, LTE that define different operating frequencies and protocols to allow user equipment (UE) to communicate with other user equipment via the telephone network. Many UEs are able to operate using a number of these different standards and typically will register with the network node that provides the best signal strength and the best service level. However, as the UE moves, it will re-register itself with different network nodes, which may operate using different technology. For example, the UE may initially be registered with an LTE network node and may move out of coverage of that node into the cell coverage of a UMTS network node. Such a handover procedure is called “inter-RAT” handover as the UE changes the Radio Access Technology (RAT) during the handover. According to the current 3GPP standards, it is the responsibility of the network nodes to determine when a UE should handover to another network node. The network node makes this decision based on measurements provided by the UE for a number of candidate neighbouring cells (defined in a neighbour cell list) provided to the UE by the network node.
It is assumed that the network nodes, such as the base stations, know the capabilities and the current status of the user equipment (UE) registered therewith, and that therefore the network nodes can manage traffic gaps (directly or indirectly, depending on the technology) to allow the UEs time to make the measurements and send the network nodes corresponding measurement reports. If the UE does not send a measurement report within the defined reporting time, the network node may assume that the candidate cells are not visible to the UE and may immediately change the list of candidate cells and sends the new cell list to the UE. The current standards require the UE to obey the latest command, and so it will discard the original cell list and will start to make measurements in respect of the new cell list.
However, a problem has been found with this procedure. In particular, if the network node overestimates the UE multi-mode capabilities or if the reported UE capabilities do not match the real time circumstances of the moment, the network node may define too short a reporting period. As a result, the UE may not have enough time to make the measurements and report the results in the defined time. The inventors have therefore, realised that strict obedience to the latest periodic measurement command could lead to a sub-optimal handover. For example, if the best cell is in the original cell list, then it would be discarded from the handover decision when the network node issues the new cell list. In future, networks are even more likely to “get it wrong” as the number of RATs increases and the combinations of UE capabilities and network features multiply, networks might not be able to predict precisely when a measurement ought to be possible by a particular UE. One solution is to give the UEs a longer reporting period, but this will mean less efficiency.